The present invention relates to a device for correcting the dislocation of a deflecting mirror included in a copier, printer, facsimile apparatus or similar image forming apparatus and, more particularly, to a deflecting mirror adjusting device advantageously applicable to a full-color image forming apparatus of the type including a plurality of image carriers implemented as photoconductive elements.
In an electrophotographic or an electrostatic image forming apparatus, errors in the dimensions of mechanisms or errors in drive cause a toner image to be formed on a photoconductive element, or image carrier, at a position deviated from an expected position, resulting in irregularity in image, distortion, and other defects. Particularly, in a color image forming apparatus which transfers images of different colors to a recording medium one above the other, the above errors bring the images out of register. The misregister or relative positional deviation between the images of different colors is conspicuous and lowers image quality noticeably. Especially, a full-color image forming apparatus of the type including a plurality of photoconductive elements involves many factors liable to cause misregister to occur. Countermeasures are most difficult to implement with this type of apparatus, as generally accepted.
Some different kinds of dislocation occur in the color image forming apparatus, as follows.
(1) Dislocation due to Shift
This dislocation is ascribable to errors in the positioning of an exposing section and photoconductive elements as well as in write timing. For example, scanning lines have their start points dislocated in the main scanning direction or are dislocated in the subscanning direction (direction of sheet feed). Further, the scanning lines are different in length due to magnification in the main scanning direction. Because this kind of dislocation is constant at any position on an image, it can be corrected if the write timing is adjusted color by color.
(2) Dislocation due to Skew
This dislocation is ascribable to errors in the parallelism of the exposing deice, photoconductive elements, transfer belt, and so forth. The scanning lines are written obliquely.
(3) Dislocation due to Curvature
This dislocation is ascribable to an error in the toroidal configuration of an f-.theta. lens. The resulting image has curvature.
(4) Dislocation due to Irregular Pitch
This dislocation is ascribable to an occurrence that irregularity in the rotation of the photoconductive elements or that of a conveyor belt causes the distance between the scanning lines to vary in the subscanning direction at the same period as the irregular rotation.
(5) Random Dislocation
This dislocation occurs abruptly without any period due to, e.g., unexpected vibration or the slippage of the transfer belt.
Generally, it is difficult to correct all of the above different kinds of dislocation with a single adjusting means. Therefore, applying a particular kind of adjusting means to each kind of dislocation is under study. As for the dislocation ascribable to skew and the dislocation ascribable to irregular pitch, it has been proposed to move a deflecting mirror at the time of image formation in order to set up an exposing position capable of cancelling the dislocation. However, the prerequisite with this kind of scheme is that the variation in dislocation be grasped beforehand, because the scheme is to correct the dislocation positively.
For the adjustment of the deflecting mirror, the mirror may be moved in the parallel direction or angularly moved. With any of the parallel movement and angular movement, it is possible to shift a focusing position on the photoconductive element. However, the angular movement is not practicable without resorting to an extremely fine resolution. Likewise, the parallel movement cannot set up a desired focusing position unless effected with extremely high accuracy.
An actuator for moving the deflecting mirror may be implemented by, e.g., a piezoelectric actuator or a drive mechanism which is disclosed in Japanese Patent Laid-Open Publication No. 7-248455 and varies the amount of feed of a screw. A piezoelectric actuator can move the mirror extremely delicately on several microns basis and can implement control with a frequency as high as several thousand hertz. However, this kind of scheme is susceptible to vibration and other disturbance because an arrangement must be so made as not to obstruct the movement control of a piezoelectric element. Moreover, because the piezoelectric element returns to its original position when a power source is turned off, its initial position must be set every time the power source is turned on. In addition, the piezoelectric actuator increases the cost. On the other hand, the screw drive scheme realizes relatively easy drive control because the displacement of the mirror is proportional to the rotation angle of the output shaft of a motor. However, the screw drive scheme also increases the cost because it needs a screw having a highly accurate pitch.
In any case, the resolution of an image to be formed on the photoconductive element depends on the accuracy of the actuator. In this respect, the configuration of the actuator is an important factor in the adjustment of the deflecting mirror. However, even if the mirror is accurately moved by the actuator, it is likely that the mirror is inclined (dislocation in the direction of normal) during movement. The inclination translates into a noticeable error in the focusing position on the photoconductive element.
It follows that accurate adjustment of movement and accurate adjustment of inclination are the important issues regarding the deflecting mirror.